Current copper foils are used not only in printed wiring board applications but as a constituting material of various types of electronic devices. With regard to the electronic devices in recent years, miniaturization is required and heat resistance against heat generation in operation of the devices is required also. Therefore, for materials constituting the electronic devices, heat resistance against high heats loaded in processing is required to improve qualities of electronic devices as final products.
For example, patent document 1 discloses object in copper foils for application in the printed wiring board technology to provide a metal-clad laminate which ensures a sufficient strength to form a stable flying lead and is applicable to a usage where fine pattern formation by using an electro-deposited copper foil. In the patent document 1, a technology in which “The metal clad laminate is formed by laminating a copper foil layer formed by the electro-deposited copper foil, a polyimide resin layer, and a stainless foil layer in this order; the average of grain size in crystal grains of copper constituting the copper layer is in the range of 0.5 to 3.0 μm, and further, difference between the average value and the maximum value of particle diameters in crystal grains is within a range of 2.0 μm or less.” is disclosed. Further, in the descriptions in claim 2 and the columns 0022 to 0024 of the patent document 1, the matter is disclosed that properties including the grain size of the crystal grains of the copper foil and physical strength as tensile strength of 400 MPa or more are necessary when improvement of the ultrasonic resistance of wiring (made of the copper foil) in the flying leads formed is investigated. Furthermore, as can be understood in the descriptions in Patent document 1, the matter is apparent that the physical strength of flying leads formed of a copper foil is required to be high even after loading a certain heat history.
Also in other technical fields, a copper foil after loading a certain heat history may be required to ensure excellent physical performance. For example, when a copper foil is used for a negative electrode current collector of a lithium ion secondary battery, the copper foil constituting the negative electrode current collector is loaded repeating expansion and contraction of an active substance provided on a negative electrode current collector.
For example, patent document 2 employs a technology to provide a current collector having high tensile strength at low cost regardless of its thin thickness that “The negative electrode current collector is characterized in provided with a hard nickel plating layer formed on at least one surface of an electro-deposited foil composed of metal material having low possibility in generation of lithium compounds. The hard plated nickel layer is formed by applying electro-plating using plating bath containing nickel, nickel salt and ammonium salt. The metal material may be an alloy of two or more selected from copper, iron, cobalt, nickel, zinc and silver, for example”. Then patent document 2 discloses that a current collector which ensures sufficiently high tensile strength even after heat treatment can be produced when the electro-deposited foil is used.
Patent document 3 discloses “The composite foils provided with a cobalt plating layer or a cobalt-nickel alloy plating layer on a surface of a copper foil are employed as a metallic foil for the negative electrode current collector of the nonaqueous electrolyte secondary batteries” to provide a composite foil which has high tensile strength even after high heat treatment and is suitably used as a negative electrode current collector of nonaqueous electrolyte secondary batteries.